If you are arriving late and just want to get to bed, I would recommend the Marriott over the Ren.

When you get your bags (or get to the pickup area), call the Marriott directly and (hopefully) they will send the shuttle right away. I have done this several times, and it is a 5-10 minute wait.

You are correct that the Marriott Shuttle picks up at each terminal (lower level).

The Ren shuttle picks up at the parking area, so you have to take the AirTrain (or whatever it is called) to the last stop, then transfer to the hotel shuttle, which only comes on a set schedule.

I'm pretty sure you can get a cab directly from the Marriott hotel, but call the front desk to double check.

I agree that the Ren is a little nicer, but for me, the additional (30+ minutes) travel time to the Ren is not worth it.

Walking to the Marriott at night could be done but personally I would wait for the shuttle since for safety (traffic) reasons.